The present invention relates to a CO sensor for measuring a concentration of carbon monoxide (CO) contained in an exhaustion gas such as from combustion engines and the like, and more particularly to a CO gas sensor which is operable at high temperatures with excluding effects of coexisting sulfur dioxide (SO2) and oxygen (O2) on CO measurement values, and a method for making the sensor and a method of using the sensor.
In boilers for thermal-power generation (oil fired boilers, LNG fired boilers, coal fired boilers, etc.) and incineration equipment, it is desired to inhibit incomplete combustion and reduce the amount of harmful CO discharged into the air. Therefore, sensors for detecting generation of CO and measuring the concentration of CO have hitherto been disposed in the above equipment.
As examples of the main CO sensors, mention may be made of semiconductor type, catalyst combustion type, solid electrolyte type, and thermal transfer type. Among them, the solid electrolyte type CO sensors utilize measurement of the electromotive force of a concentration cell as a principle of measurement, and it comprises a metal electrode and a stabilized zirconia having oxygen ion conductivity and is superior in heat resistance, shock resistance and poisoning resistance.
Furthermore, since exhaustion gases from various equipment such as boilers for thermal-power generation contain harmful components such as NOx and SO2, a duty is imposed in the equipment to monitor the concentrations of these harmful gases in the exhaustion gas, and a non-dispersion infrared absorption type (NDIR type) measurement apparatus is mainly used for these gases. This apparatus is not directly inserted in the exhaustion gas, but the exhaustion gas is subjected to sampling by an absorption pump and analysis is conducted at a place at a distance from a gas flue (passage of exhaustion gas)
The CO sensors using the stabilized zirconia sense CO utilizing the oxidation reaction of CO, and in case the detection gas contains gases such as SO2, these gases are also further oxidized and sensed. Thus, there is the problem that accuracy for the measurement of CO concentration is lowered. Although detection sensitivity for SO2 and others is lower than that for CO, if the concentrations of these gases increase, measurement error increases. Therefore, in order to perform a precise measurement of CO concentration, the measurement Aceq value of CO must be corrected based on the information obtained from the NDIR type sensor.
However, measurement using these different sensors is inconvenient in installation of the apparatuses, maintenance and inspection, and data processing. In the case of NDIR type sensor, there are further problems that a time lag is caused by collection and analysis of gas, and precision of measurement is lowered due to the influence by other gases such as CO, carbon dioxide (CO2) and hydrocarbons (HC). Of course, there is a problem of being affected by NOx or SO2 when CO is measured by NDIR.
Furthermore, the solid electrolyte type CO sensor suffers from the further problem that since platinum is used as an electrode material, it also works as an oxygen (O2) sensor, and the CO concentration cannot be selectively measured for the gas in which O2 coexists. In this case, at high temperatures of 600-900xc2x0 C., the oxidation reaction of CO rapidly proceeds on the surface of the platinum electrode, and when oxygen is contained in the gas to be measured, it is difficult to distinguish the measurement results attributable to oxygen and CO from each other.
The present invention has been accomplished in an attempt to solve the problems in the conventional techniques, and the object is to provide a CO sensor using a solid electrolyte for precise measurement of CO concentration which simultaneously carries out measurement of the concentration of SO2 and/or O2 which are especially liable to cause measurement error while excluding the influence of SO2 or O2.
That is, according to the present invention, there is provided a carbon monoxide sensor comprising a solid electrolyte having oxygen ion conductivity, a standard electrode and a sensing electrode for measurement of carbon monoxide, and one or more standard electrodes and sensing electrodes for measurement of one or more different gases other than carbon monoxide, said standard electrodes and sensing electrodes being formed on at least a part of the surface of the solid electrolyte, characterized in that one of the different gases is sulfur dioxide.
Furthermore, according to the present invention, there is provided a method for making a carbon monoxide sensor which comprises a bottomed cylindrical or rod-like solid electrolyte having an electrode on the outer surface of the bottom portion and provided with at least one bottomed hole at the bottom portion inside the electrolyte, an electrode being provided on the inner surface of the bottomed hole, characterized in that the bottom portion of the solid electrolyte is dipped in an organic metal solution containing Au or a component of Au alloy and/or a component of the solid electrolyte, and/or the organic metal solution is poured into the bottomed hole and sucked out therefrom to coat the inner surface of the bottomed hole with the solution, followed by firing to form the electrodes.
According to the method for making a carbon monoxide sensor of the present invention, not only the production cost can be reduced by a simple production method depending on the shape of the desired carbon monoxide sensor, but also the quality of the sensor produced can be maintained at constant.
Furthermore, the present invention provides a method of using a carbon monoxide sensor for measurement of carbon monoxide concentration, said carbon monoxide sensor comprising standard electrodes and sensing electrodes for measurement of carbon monoxide concentration and for measurement of sulfur dioxide concentration, respectively, and/or reference electrodes for measurement of carbon monoxide concentration and for measurement of sulfur dioxide concentration, respectively, and/or a standard electrode for measurement of oxygen concentration, and/or a sensing electrode for measurement of oxygen concentration which are formed on at least a part of the surface of a solid electrolyte having oxygen ion conductivity, characterized in that the carbon monoxide concentration and the sulfur dioxide concentration are simultaneously measured, and the carbon monoxide concentration is determined by correcting the result of measurement of the carbon monoxide concentration using the result of measurement of the sulfur dioxide concentration.
As a suitable method, mention may be made of a method in which the carbon monoxide concentration is measured by measuring change in electromotive force between the sensing electrode and the standard electrode and/or change in electromotive force between the sensing electrode and the reference electrode which are caused by adsorption/oxidation of carbon monoxide when a constant current is allowed to flow between the sensing electrode and the standard electrode for measurement of carbon monoxide, and the sulfur dioxide concentration is measured by measuring change in electromotive force between the sensing electrode and the standard electrode and/or change in electromotive force between the sensing electrode and the reference electrode which are caused by adsorption/oxidation of sulfur dioxide when a constant current is allowed to flow between the sensing electrode and the standard electrode for measurement of the sulfur dioxide concentration.